Blood coagulation or clotting is the result of a complex series of biochemical reactions. In the normal course of events, hemostasis and the associated process of blood coagulation prevent undue loss of blood from an injured blood vessel. However, inappropriate coagulation of blood (thrombosis) may occur within the circulatory system in pathological states such as atheroschlerosis or in response to a variety of insults, including surgery and implantation of medical devices. Such inappropriate clotting results in thrombus formation, which may cause occlusion of a vessel and/or thromboembolism, in which all or part of a blood clot breaks loose and becomes lodged as an embolus in another region of the circulatory system. Such emboli are, in some cases, life-threatening, especially when they cause obstruction of the pulmonary or cerebrovascular circulatory system.
Prevention of thrombosis is considered a crucial part of the treatment regimen for patients at risk for developing thrombi. Disease or treatment states in which antithrombotic therapy is indicated include replacement of heart valves, grafting procedures, chronic bedrest, surgery, venous thrombosis and pulmonary embolism, arterial embolism, stroke, presence of abnormal coagulation factors, certain stem cell diseases, and homocystinuria.
In order to understand the various means by which blood coagulation, and, consequently, thrombosis, can be controlled, a basic understanding of the cascade of reactions leading to formation of fibrin and blood clots within the circulatory system is essential. These reactions and their components have been reviewed extensively (Majerus,Baboir) and will be only summarized briefly with reference to FIG. 29 herein.
Coagulation of blood can be stimulated by either of two different, but interconnected pathways--the intrinsic and extrinsic pathways. In both pathways, blood coagulation results from a series of zymogen activation steps involving enzymatic cleavage of the inactive zymogen molecule to an active protease, which, in turn, activates the next enzyme in the pathway. With reference to FIG. 29, the linking point between the intrinsic and extrinsic pathways is activation of the zymogen Factor IX to the active protease, Factor IXa.
The intrinsic pathway is so called, because, following the initial contact stimulus, only factors intrinsic to the blood are involved in its functioning. In this pathway, as studied in vitro, interaction of Factor XII, prekallikrein, and high molecular weight kininogen with a foreign surface, such as glass or kaolin, results in conversion of Factor XII to Factor XIIa, which in turn activates Factor IX to Factor IXa.
Factor IXa is a protease which converts inactive Factor X to active Factor Xa. This conversion is accelerated by the presence of platelets or phospholipids (both designated PL in the figure), cofactor VIIIa, and calcium. The conversion of Factor II (prothrombin) to form Factor IIa (thrombin) is enhanced by the presence of platelets or phospholipids, factor Va, and calcium. Factor Va can be released by stimulated platelets.
Thrombin is a protease which cleaves the high molecular weight fibrinogen to fibrin monomers. These monomers form a gel, to which red blood cells adhere to form a blood clot. The strength of the clot is increased by the fibrin monomer interchain transglutamination reactions, catalyzed by factor XIIIa.
To complete the common pathway shown in FIG. 29, clots are broken down ("dissolved") by an endogenous fibrinolytic system. The active protease plasmin is formed from inactive plasminogen by enzymatic cleavage catalyzed in vivo by one or more of a number of endogenous activators, including tissue plasminogen activator (t-PA). Streptokinase, a bacterial product, or urokinase, isolated from human cells, are also capable of activating plasminogen. Plasmin non-specifically cleaves fibrin and other plasma proteins, including some of the clotting factors.
In the extrinsic pathway, exposure of blood to a tissue factor is the stimulus for conversion of Factor IX to Factor IXa. Tissue Factor is a lipoprotein present on surfaces of non-circulatory cells, such as fibroblasts or smooth muscle cells to which the blood may be exposed in certain pathological states. As shown in FIG. 29, Factor VIIa, in the presence of calcium, effects the conversion of Factor IX to Factor IXa as well as the conversion of Factor X to Factor Xa. Factor VII itself has about 1/100 the proteolytic activity of Factor VIIa, and is therefore able to initiate clotting. Tissue factor increases the activities of both Factor VII and Factor VIIa about 30,000 fold. Formation of Factor Xa, also accelerates the process by converting still more Factor VII to Factor VIIa.
In general, agents which affect blood hemostasis fall into three categories: agents which interfere with portions of the above-described coagulation cascade (anticoagulants), agents which interfere with platelet activation and aggregation (antiplatelet drugs), and agents which promote disintegration of blood clots (thrombolytics). Anticoagulants and antiplatelet drugs are categorized as antithrombotics, used in preventing and arresting thrombus formation in arterial and venous blood vessels, as described above. Antiplatelet agents interfere with the initial stages of platelet aggregation initiated by contact of platelets with collagen, such as occurs during blood vessel damage. These agents are used clinically in prophylaxis of arterial thromboses, such as occur in atheroschlerosis. Anticoagulant compounds, by interfering with the clotting cascade, inhibit those components of clot formation associated with fibrin deposition, and are more generally used in prevention of venous thromboses. Thrombolytic agents are used in dissolution of formed thrombi in both venous and arterial vessels.
Aspirin, dipyridamole and ticlopidine are examples of antiplatelet drugs. These agents are generally used in prophylaxis of arterial thrombus formation as in atherosclerotic disease, repeat myocardial infarction, transient ischemic attack, and alone or in association with anticoagulants in certain cardiac valvular disorders. They are not generally used in the treatment of other abnormal clotting events, such as venous thrombosis, nor is there considered to be a mechanistic basis for their use in such disorders.
Agents which promote disintegration of blood clots (fibrinolytic agents) include tissue plasminogen activator, streptokinase and urokinase. These compounds are used post-myocardial infarction to prevent thromboembolism.
Currently available anticoagulant drugs are limited to the heparin-like compounds, which are active only when given intravenously, and to the oral coumarin anticoagulants. Heparin is an endogenous glycosaminoglycan which serves as a catalyst for the reaction between antithrombin and various activated factors in the coagulation cascade (Factors IXa, Xa, XIa, XIIa, kallikrein and thrombin). This reaction results in inhibition of these factors, and thus inhibition of coagulation. Heparin is not well absorbed orally and has a relatively short half-life in the bloodstream. Side effects of long term heparin therapy can include thrombocytopenia with associated paradoxical arterial thrombosis, and, rarely, osteoporosis. Overdosage with heparin can be antagonized by injection of protamine sulfate.
Oral anticoagulants, including warfarin and other coumarin derivatives, produce their effects on blood coagulation by indirect means. These compounds inhibit regeneration of vitamin K in the liver. Vitamin K is a precursor to several of the coagulation pathway factors, including Factors II (prothrombin), VII, IX, and X; therefore, depletion of vitamin K results in inhibition of coagulation. As might be expected from their mechanism, the coumarin drugs have a relatively long onset of therapeutic activity, since their effectiveness is dependent upon depletion of endogenous depots of active vitamin K. Coumarin therapy requires careful management, due to a number of drug and nutritional interactions which serve increase or decrease effective dosage levels. Treatment with coumarin derivatives is also associated with several serious side effects including bleeding episodes and teratogenicity.
A number of analytical tests have been devised to measure the patency of the above-described coagulation cascade. These tests, which are described in more detail below, are generally carried out on blood plasma. Specific assays have been developed to distinguish the particular sites of activity of the various compounds in the clotting cascade and to distinguish between their overall anticoagulant effects and their antithrombotic activity. For example, the prothrombin time assay (PT) measures the extrinsic system of coagulation and is therefore used to detect deficiencies in factors II, V, VII, and X. PT is also used to monitor therapy in patients receiving coumarin anticoagulants, since factors II and VII are among those which are dependent upon vitamin K. The activated partial thromboplastin time assay (APTT) measures coagulation factors present in the intrinsic system of coagulation and is generally used to monitor heparin therapy. Antiplatelet activity can be measured directly in plasma samples.
As described above, current anticoagulant regimens include treatment with various forms of heparin, or coumarin drugs. Of the two, the heparin drugs are by far the better tolerated and are easier to titrate. However, the usefulness of these compounds is limited by their currently obligatory intravenous route of administration. Although formulations of these compounds have been administered enterally, anticoagulant activity has been observed only after intraduodenal administration (Andriuoli, Caramazza).
Coumarin drugs such as warfarin can be given orally; however, the usefulness of these drugs is limited by their relatively long onset time, difficulty in titration, interactions with other drugs, and side-effects, as described above. In addition, both heparin and coumarin drugs are limited in their usefulness by their hemorrhagic potential, because effective antithrombotic doses of these drugs also concommitantly produce excessive systemic anticoagulation.
It is therefore a general object of the present invention to provide compounds and methods for oral anticoagulant therapy with a shorter onset and duration of action, for improved oral antithrombotic activity. A further object of the invention is to provide antithrombotic agents having reduced hemorrhagic effects, at antithrombotic doses.